The present invention relates generally to floating roof tanks and more specifically to methods and apparatus for sealing the guide pole opening in the floating roof to reduce emissions of vapor from the tank.
Ambient air quality has become an increasingly important concern in recent years. Many air pollutant emission sources that were tolerated in years past are now facing regulations which force significant reductions or elimination of such emissions. One category of such emission sources is aboveground storage tanks for the storage of volatile liquids.
Although there are other types of aboveground storage tanks for the storage of volatile liquids, one type of such tank in wide use is referred to as an external floating-roof tank. This type of tank has a circular essentially flat bottom, a vertical cylindrical shell having a lower edge joined to the tank bottom and an external floating roof adapted to float on the volatile liquid stored in the tank. The rim space, which is located between the floating roof rim and the inside surface of the tank shell, is sealed by one of several rim sealing means attached to and movable vertically simultaneously with the floating roof so as to reduce emissions to the atmosphere from the rim space. Some such seals are disclosed in the U.S. Pat. Numbers: Moyer U.S. Pat. No. 2,829,795; Harris et al. U.S. Pat. No. 2,968,420; Reese U.S. Pat. No. 3,075,668; Wissmiller U.S. Pat. No. 3,120,320; Moyer U.S. Pat. No. 3,136,444; and Bruening U.S. Pat. No. 4,406,377.
The floating roof moves vertically upward when the storage tank is filled with product, and moves vertically downward when product is withdrawn from the storage tank. Although the external floating roof is permitted to move in a vertical direction and, to a lesser extent, in a radial direction, it is necessary to provide guides to prevent rotation of the floating roof so as to prevent damage to other appurtenances on the floating roof such as rolling ladders, rainwater drain systems, and automatic level gauges.
To prevent rotation of the floating roof, a guide pole is commonly used. The guide pole is located inside of the storage tank near the tank shell and is fixed at the bottom to the tank bottom and is fixed at the top to the top of the tank shell. The guide pole penetrates the floating roof through a guide pole fitting, which results in a source of emissions to the atmosphere.
Gauging the product liquid level in the storage tank or obtaining samples of the product in the storage tank has been done utilizing the interior of the guide pole. To facilitate gauging and sampling operations, the guide pole is hollow and has openings to allow the product inside of the guide pole to freely mix with product outside of the guide pole so that the composition and liquid level inside of the guide pole are the same as that outside of the guide pole in the storage tank. These openings are often in the form of vertical columns of slots which overlap on alternating rows so that at any vertical position there is always communication between the liquid within the guide pole and the liquid outside of the guide pole.
The wind has been found to have an important effect in causing emissions from certain types of roof fittings and wind tunnel tests have been performed to measure the emission loss factors of different types of floating roof fittings, including guide pole fittings. A wind tunnel simulated the flow of atmospheric air over the floating roof fittings, as occurs on external floating roofs, and revealed that the guide pole fitting had the highest emissions of all of the fittings tested. In fact, one type of commonly used guide pole fitting had emissions that were about 25 times the emissions from the entire rim seal of an external floating roof.
Therefore, it is desirable to incorporate emission control features in guide pole fittings to reduce the emission loss factors.